1. Field of the Invention
The present invention relates to a high-frequency waveguide and a method of manufacturing it, and particularly to a waveguide through which electromagnetic waves lying in a microwave band, a millimeter-wave band and a submillimeter-wave band propagate, and a manufacturing method thereof.
2. Description of the Related Art
As a waveguide for allowing electromagnetic waves (hereinafter called “high-frequency waves”) lying in microwave, millimeter-wave, and submillimeter wave bands to propagate, a hybrid waveguide comprising a combination of wave guides, metals and a dielectric have been used. An NRD (nonradiative dielectric) guide with a dielectric interposed between two metal plates has been used as a waveguide in which metals and a dielectric are utilized in combination. As the known references, there are IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-29, NO. 11, NOVEMBER 1981, PP. 1188-1192, and IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-32, NO. 8, AUGUST 1984, PP. 943-946.
While the NRD guide has the feature that no radiation loss is produced at a bent portion of a waveguide, propagation loss increases because it is used in the neighborhood of a cutoff frequency of the waveguide. In addition to this, a waveguide using a photonic band crystal structure has been placed under study as a waveguide low in radiation loss.
The photonic band crystal structure includes an artificial crystal having a dielectric periodic structure having a high dielectric constant ratio and allows the occurrence of such an event that the propagation of energy is prohibited, at a given energy region, in the same manner as the case where the crystal controls electrons. The formation of a periodic-structure disturbing portion at part of the photonic band crystal structure makes it possible to cause energy to propagate through such a defective portion alone, whereby it can be formed as an energy propagation path.
As the known reference wherein the photonic band crystal structure is formed as a waveguide for optical transmission, there is known NATURE, VOL. 386, 13 Mar. 1997.
Further, Japanese Patent Application Laid Open No. 2000-352631 describes photonic crystals and a method of manufacturing the same. This shows one wherein cylindrical dielectrics arranged in a triangular lattice form to increase a mechanical strength are utilized in combination with perfect band gaps comprising dielectrics two-dimensionally arranged in a honeycomb lattice form as photonic crystals used in the field of the optical transmission.
Furthermore, Japanese Patent Application Laid-Open No. Hei 11(1999)-218627 describes a photonic crystal waveguide and a method of manufacturing it. This shows one formed with a slab optical waveguide formed of quartz glass or a polymeric material on a silicon substrate as a photonic crystal waveguide used in the field of optical communications. The slab optical waveguide is one wherein materials different in refractive index are arranged on both sides of a centrally-provided optical waveguide area in the form of a triangular lattice or a hexagonal lattice to provide refractive index variation areas. However, these photonic crystal waveguides are techniques related to optical waveguiding.
FIG. 7 is a perspective view of a conventional high-frequency waveguide based on a photonic band structure.
In FIG. 7, reference numeral 100 indicates a high-frequency waveguide, reference numeral 102 indicates a dielectric such as ceramic, and reference numerals 104 respectively indicate air columns whose arrangements in this air constitute a photonic band crystal structure. Reference numerals 106 indicate metal plates bonded to each other at both end faces of the dielectric 102 as viewed in the direction orthogonal to the air columns 104. In FIG. 7, the metal plates 106 are hatched as being not intended to indicate their sections but indented to clearly define a relationship of position between the two metal plates 106 and the dielectric 102.
FIG. 8 is a cross-sectional view of the high-frequency waveguide 100 as viewed from a section thereof taken along line VIII—VIII of FIG. 7. The section taken along line VIII—VIII corresponds to a section orthogonal to each of the air columns 104.
In FIG. 8, reference numerals 108 indicate high-frequency reflecting areas, and reference numeral 110 indicates a high-frequency propagation area.
When a high-frequency wave propagates through the high-frequency waveguide 100, each of the high-frequency reflecting areas 108 prohibits the propagation of a high-frequency wave corresponding to the photonic band crystal structure. However, since the high-frequency propagation area 110 has no air columns 104 and results in a defect of the photonic band crystal structure, the high-frequency wave can propagate through this portion.
When an electromagnetic wave propagates through the high-frequency propagation area 110, a high-frequency current flows due to an omnidirectional magnetic field as viewed in the tangential direction of each metal plate 106. This results in transmission loss of Joule's heat. However, since the transmission loss decreases with an increase in frequency in a mode in which the magnetic field principally has a high-frequency transmission direction of the high-frequency propagation area 110, it normally presents no problem.
However, since the high-frequency propagation area 110 makes use of a dielectric high in dielectric constant, a dielectric loss increases significantly.
FIG. 9 is a perspective view of a conventional high-frequency waveguide based on another photonic band structure. The same reference numerals as those shown in FIGS. 7 and 8 respectively indicate the same or equivalent ones. Even in the case of the description of the following drawings, the same reference numerals respectively indicate the same or equivalent ones.
Reference numeral 112 indicates a high-frequency waveguide, and reference numerals 114 and 116 respectively indicate dielectrics such as ceramic.
FIG. 10 is a partly sectional view of the high-frequency waveguide 112 as viewed from a section thereof taken along line X—X of FIG. 9, and FIG. 11 is a cross-sectional view of the high-frequency waveguide 112 as viewed from a section thereof taken along XI—XI of FIG. 9, respectively.
In the high-frequency waveguide 112 of FIGS. 10 and 11, high-frequency reflecting areas 108 are disposed in parts as two independent portions in which air columns 104 are regularly arranged in the dielectrics 114 and 116. A high-frequency propagation area 110 is defined as space filled with air. Therefore, a dielectric loss at this portion can be reduced.
However, in either case of the high-frequency waveguide 100 of FIGS. 7 and 8 and the high-frequency waveguide 112 of FIGS. 10 and 11, it is difficult to carry out the work of forming the desired air columns 110 in the dielectrics. Since the high-frequency propagation area 110 is defined in the space in the high-frequency waveguide 112, it is difficult to carry out dielectric-removing processing. This is not suited to mass production.
On the other hand, the paper Vol. J84-C No. 4 pp. 324-325, April 2001 issued by the Institute of Electronics, Information and Communication Engineers has described a photonic crystal waveguide wherein columnar bars in which alumina is covered with styrofoam, are provided in a triangular lattice array. However, this will cause an increase in loss.